Photophysics of Polymers - American Chemical Society

Chapter 17

Photophysics of 1,5-Naphthalene Diisocyanate-Based Polyurethanes Charles E. Hoyle and Kyu-Jun Kim

Downloaded by COLUMBIA UNIV on June 6, 2017 | http://pubs.acs.org Publication Date: November 30, 1987 | doi: 10.1021/bk-1987-0358.ch017

Department of Polymer Science, University of Southern Mississippi, Hattiesburg, MS 39406-0076

Using both steady-state and transient fluorescence decay spectroscopy, the formation of intramolecular excimers in dilute solution of a naphthalene diisocyanate based polyurethane is identified. Investigation of an appropriate model compound leads to the conclusion that hydrogen bonding is a key factor in stabilizing excimers formed from naphthyl carbamates. While the decay kinetics of the model naphthyl carbamate are described by a typical Birks excimer scheme involving a single excited species in dynamic equilibrium with the excimer, the polymer decay kinetics can only be adequately interpreted by an "isolated monomer" scheme involving both an interactive (excimer forming) and non-interactive (isolated monomer) excited naphthyl carbamate moiety. The extent of excimer formation is dependent on the ability of the solvent to solvate the polyurethane, i.e., excimer formation is increased in poor solvents. In addition, excimer formation in solid polyurethane films is quite high where hydrogen bonding, as identified by the shift in the carbonyl stretching frequency, is prevalent. Excimers, which are simply excited state complexes formed from e q u i v a l e n t chemical species, one of which i s excited p r i o r t o complexation, were f i r s t r e p o r t e d i n s m a l l molecule systems. However, during the past 20 years one of the more vigorous research areas i n photophysics has been the investigation of excimers formed i n polymers (1). In most of the cases reported to date, the excimer studies i n polymer systems have been conducted on polymers bearing pendant aromatic chromophores. Only a few papers have been published on excimers formed from polymers with the two species p a r t i c i p a t i n g i n excimer formation spaced a t r e l a t i v e l y large

0097-6156/87/0358-0201 $06.00/0 © 1987 American Chemical Society

Hoyle and Torkelson; Photophysics of Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

PHOTOPHYSICS OF POLYMERS

202

i n t e r v a l s from each other along the backbone (2-5). Herein, we report on intramolecular excimers between naphthyl carbamate groups spaced p e r i o d i c a l l y i n polyurethanes made from 1,5-naphthalene diisocyanate. I t i s found that even i n very d i l u t e solutions, well below the concentrations required for intermolecuiar interaction, excimer emission can be quite strong, depending on the nature of the s o l v a t i n g system employed. The d r i v i n g mechanism for excimer formation of the naphthyl carbamate groups i s shown to be based, at least i n part, on hydrogen bonding. EXPERIMENTAL

SECTION


Materials

Dichloromethane, dimethylformamide (DMF), and benzene were obtained from Burdick and Jackson and used without further p u r i f i c a t i o n . 2,3-Butanediol and 1-butanol (Aldrich) were used as received. Propyl benzene (Aldrich) was d i s t i l l e d before use. Deionized water was used. Equipment

Emission spectra and absorption spectra were recorded on a PerkinElmer 650-10S Fluorescence Spectrophotometer and a Perkin-Elmer 320 UV Spectrophotometer, respectively. Fluorescence decay data were obtained on a single-photon-counting apparatus from Photochemical Research Associates. The samples were bubbled with nitrogen f o r the s t e a d y - s t a t e f l u o r e s c e n c e s p e c t r a and the fluorescence decay measurements. In some cases, front face spectra were taken. The data were analyzed by a software package from PRA cased on the i t e r a t i v e convolution method. C NMR spectra were obtained on a JEOL FX90Q, and FTIR spectra were recorded on a Nicolet 5DX. The elemental analyses were conducted by M-K-W Laboratories of Phoenix, AZ. 1 3

Synthesis of

Model Compounds

1,5-Naphthalene diisocyanate ( NDI ). To a s t i r r i n g s o l u t i o n of pdioxane (50 mL) containing the i,5-diaminonaphthalene (Fluka, 5.1 g) was added trichioromethyl chloroformate (Fluka, 17 g) i n p-dioxane (15 mL) through an addition funnel under a nitrogen stream. A white p r e c i p i t a t e was immediately observed. After addition, the temperature was increased to reflux. The forming HC1 was removed by passing through water. After 1 hour, the s o l u t i o n turned clear and was a l l o w e d t o react for another 3 hours. The p-dioxane was evaporated under reduced pressure and the r e s u l t i n g s o l i d was vacuum sublimed twice to give c o l o r l e s s c r y s t a l s i n 71% y i e l d : πτρ> 126128°C ( l i t . mp 129.5-131°C) ; IR 3022, 2300, 1600, 1500 cm"""; C NMR 130.9, 128.7, 127.7, 124.2, 122.2 ppm (benzene); A n a l . 12 6 2°2 - ' · < · ' · ex

/

e

m

= 330


17.

HOYLEANDKIM

207

( r ) and 15.3 ns ( τ ) . By monitoring at 480 nm in the excimer region and f i t t i n g the long-lived portion of the decay curve to a s i n g l e exponential decay function, a l i f e t i m e of 15.4 ns (Table I) was recorded. Comparison of the s h o r t - l i v e d component (0.33 n s — τ2 ) of the monomer decay i n the concentrated s o l u t i o n with the l i f e t i m e (3.53 n s — r ) i n the d i l u t e s o l u t i o n leads to the conclusion that the monomer emission i n the concentrated s o l u t i o n i s s i g n i f i c a n t l y quenched. Both the large decrease of the fluorescence l i f e t i m e of the monomer naphthyl carbamate and the presence of the long-lived component i n the monomer emission region suggests that the monomer e x i s t s i n dynamic equilibrium with the excimer as represented by the c l a s s i c a l scheme (Scheme I) for excimer k i n e t i c s . The value for k j ^ has a f i n i t e value which makes the d i s s o c i a t i o n of the excimer E* into i t s component species M and M* a v i a b l e process. At t h i s point, i t i s appropriate to consider the factors which may enter into s t a b i l i z a t i o n of the excimer i n the ?NC solutions. F i r s t , i t i s worth noting that at least some excimer formation, as monitored by the recording of fluorescence decay curves i n the monomer region, occurs at concentrations of ?NC as l i t t l e as 0.005 M i n organic solvents. However, i t i s o n l y above 0.5 M that appreciable baiId up of excimer emission i s recorded. It has long been recognized that polyurethanes, which contain carbamate chromophores, are c h a r a c t e r i z e d by hydrogen bonding between the hydrogen attached to the nitrogen of the carbamate group and the carbamate carbonyi (or an ether group i f present). This contributes to the unique physical properties of polyurethanes. Unsurprising, the PNC model system a l s o shows appreciable hydrogen bonding which i s p r e v a l e n t a t h i g h e r c o n c e n t r a t i o n s between carbamate chromophores. This i s i l l u s t r a t e d i n Figure 4 by the IR s p e c t r a of ?NC recorded at two concentrations. In the d i l u t e solution (0.2 M) the IR shows a s i n g l e peak i n the N-H stretching region. This peak at 3416 cm""- r e s u l t s from a non-bonded (or free) N-H stretching i n the carbamate moiety. At the higher concentration (1.5 M) a new band due to a hydrogen-bonded N-H stretching appears at 3320 cm""*. I t i s i n t h i s concentration region that the higher degree of excimer formation i s recorded i n Figure 2. Thus, i t i s reasonable to assume that hydrogen bonding between the PNC molecules contributes to the s t a b i l i z a t i o n of the excimer. In support of t h i s supposition, propyl N-methyl N-(1-naphthyl) carbamate (PNMNC) with a methyl group substituted on the nitrogen carbamate was synthesized. The methyl group p r o h i b i t s hydrogen bonding. Figure 5 shows that l i t t l e or no excimer emission i s recorded f o r PNMNC up to concentrations of 2.2 M i n dichloromethane. Only at concentrations above 3 M can any excimer emission be observed. Consequently, i t can be concluded that hydrogen bonding i s indeed an important factor i n the excimer formation of ?NC. 2


Naphthalene Diisocyanate-Based Polyurethanes

3

1

NÎCH )C0 Pr 3

2

PNMNC


PHOTOPHYSICS OF POLYMERS

208

Scheme I

k

M-

D M

k


[Ml

M MD

Μ+ηυ'

2Μ+ηυ"

+ heat + product

k

M

=

non-radiative excited

^DM

=

r

a

t

e

^MD

=

r

a

t

=

non

rate

constant

for

for

excimer

formation

for

dissociation

between

M and

.

e

constant

component kg

radiative

PNC monomer M * .

constant

M

plus

+ heat + product

radiative

species plus

of

excimer

Ε

M and M * .

radiative

rate

constant

for

excimer E * . M =

ground

s t a t e PNC.

M

=

excited

E*

=

PNC e x c i m e r .

into

s t a t e PNC.


17.

H O Y L E AND KIM

Naphthalene Diisoeyanate-Based

Polyurethane*

ω ο c (Ό


Photophysics of Polymers - American Chemical Society

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